I had many things with me in Namibia… I’ll only mention a few things that either surprised me how useful were or I think that some people might not have them.
Basically one can download countries off-line (like Namibia) and it’s based on OpenStreetMap data. In Namibia the OpenStreetMap data was more than enough and in some areas it was amazing! (for example it had the small walking paths in Okonjiima, with the names and everything).
OsmAnd has some usability problems so if you want to use it learn it before the trip and add as favourites the places that you are going to sleep and other interesting points.
A real paper map is still useful for planning and navigation but I think that OsmAnd (or some other off-line app) really helps to calculate times and distances, I like seeing the Km. left and Estimated Time of Arrival and I find the fact that are the cities as well very useful. Also with paper maps one would need more maps and guides.
Other option could be MapsMe, I haven’t tried this one.
Probably this is obvious to many people but initially I didn’t consider bringing binoculars. A week before going to Namibia I tested some in Europe and thought that could be useful and actually they were absolutely essential! I had the typical 8×40 (Olympus 8×40).
Actually I considered using very old ones, from when I was a child… and compared with a “proper ones” my old ones were making the objects smaller! It’s also important that they are wide-range so you can see many things at once.
Animal field guides
I’m sure that for many people a field guide is an obvious thing to have. Initially I didn’t consider bringing thick books for mammals, birds and snakes/insects (yes, 3 books!). I thought that this was insane. But once there I borrowed the books often, I read the descriptions and I played my game of «guessing the bird here» (if the bird was keen enough to pose for me about 10 minutes while I was browsing the book).
Sitting there, seeing an animal and then being able to pint-point which type of zebra (or other) was very interesting. Then my favourite bit was reading their behaviour: what they do, when they do, are they nocturnal? Are they are in a pack? How do they hunt? What do they eat? Do they hibernate? Migrate?